1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a nonvolatile memory cell and a method of manufacturing the same.
2. Discussion of Related Art
The advancement of silicon-based material and device technology, i.e., silicon electronics, has led to the development of the electronic industry. However, the silicon electronics may include hard, breakable materials and be opaque in the visible light region. In recent years, in order to overcome the restrictions of the silicon electronics, flexible electronics in which electronic devices and systems are manufactured on a flexible substrate and transparent electronics in which transparent electronic devices and systems are manufactured have been proposed. In addition, research and development are being conducted on various applications, such as sensors, displays, electronic circuits, and batteries.
In the field of transparent electronics, a transparent thin-film transistor (TFT) technique and a transparent display technique using a transparent TFT as a driver circuit are rapidly being developed. Currently, technical development enters into a step of raising technology maturity for putting the transparent TFTs and displays to practical use and a step of designing target applications in order to realize transparent electronic circuits on a substrate using a driver transistor.
However, technical development of transparent devices (i.e., driver transistors) for displaying and processing data is briskly progressing, while technical development of memory transistors for storing data is falling behind. Since a memory device may be mounted outside a system, it may be less necessary to make the memory device transparent. However, by mounting the memory device inside the system, it may be more effective to control the functions of the memory device and reduce not only power consumption but also the cost of the mounting of the memory device.
Thus, the following points may be required to mount the memory device inside the system.
First, the memory device may be a nonvolatile memory device.
Memory devices may be classified into volatile memories and nonvolatile memories depending on how to store data. The volatile memories may store data only during the supply of power, while the nonvolatile memories may store data even if power is interrupted. Since transparent electronics are highly likely to embody a stand-alone electronic device to which power is always supplied or a design-oriented application with a mobile function, the functions of the nonvolatile memory device may be required to increase the lifespan of batteries and retain capability to store a large amount of data.
Second, an operating voltage of the memory device may be within a predetermined range. When an excessively high operating voltage is required for a memory operation in consideration of only transparency, the entire system cannot resist to the high voltage and it may become unnecessary to mount the memory device within the system or an integrated circuit (IC). Furthermore, the memory device should be capable of stable operations within the range of an operating voltage of a module used together with the memory device.
Third, the size of the memory device should not be excessively large. A memory transistor for a transparent electronics system may be not only a data storage device but also an embedded memory of the system. Thus, the size of the memory transistor may be minimized to downscale the entire system.
Fourth, high device stability appropriate for the required operation of a system should be ensured. A nonvolatile memory device should have tolerance to repeated write operations, that is, a good cycling characteristic. Also, the nonvolatile memory device should have a good data retention characteristic. Furthermore, the nonvolatile memory device should be highly capable of retaining stored data under high-temperature or humid conditions. Although a memory device used for transparent electronics may not satisfy high reliability required for typical silicon electronics, the memory device used for the transparent electronics needs to satisfy reliability specification required by the corresponding application.
Conventionally, in order to provide a nonvolatile memory that meets the foregoing requirements and exhibits transparency and flexibility, the following operating principles of the memory transistor may be provided.
First, a transparent oxide layer having a large energy bandgap may be used, and the resistance of the oxide layer may be varied with the application of a voltage. That is, the resistance of the oxide layer may be varied according to the magnitude or direction of a voltage applied to the oxide layer so as to store data. A device using the above-described method may be typically referred to as an oxide resistive memory device, which has been highlighted as an advanced nonvolatile memory device that will replace a flash memory device.
In order to apply the above-described operating principles to transparent electronics, components of the entire device need to be formed of transparent materials. Thus, an oxide layer, which is an essential component of a resistive memory device, should be formed of a material that has a large bandgap and experiences a wide range of resistance variation according to the magnitude or direction of an applied voltage.
Since an oxide resistive memory device has a relatively simple structure, an area occupied by the entire memory may be greatly reduced. However, it is known that the principle on which the resistance of the oxide resistive memory varies with the magnitude or direction of the applied voltage was not completely revealed, and the characteristics of the oxide resistive memory are greatly varied according to materials of upper and lower electrodes. That is, it may be difficult to uniformize the characteristics of devices and predict a variation of device characteristics relative to process variations. Also, since the operating principles of the oxide resistive memory are not clearly disclosed, adopting the oxide resistive memory as an embedded memory transistor of the system may be difficult.
Second, a charge storage region may be prepared in a predetermined portion of a memory device so that a threshold voltage of a transistor may be varied according to the magnitude and direction of an applied voltage to embody a memory operation. The charge storage region may be a thin layer constituting a portion of a gate of the transistor or nanodots. In general, this technique may be referred to as a nano-floating-gate memory that is developed as a portion of advanced flash memory technology in conventional silicon electronics. By adding a process of preparing the charge storage region in a partial region of a gate sack while using the structure of a transparent or flexible TFT as it is, a memory device may be manufactured using a relatively simple process.
However, since an oxide or organic material is used as a semiconductor material, it may be more difficult to quantitatively control the storage of charges than when silicon semiconductor is used. Also, an oxide or organic semiconductor TFT using an accumulation layer and a depletion layer cannot reduce a required voltage due to its driving characteristics.
Third, a memory device may adopt an organic thin layer with predetermined characteristics, and the resistance of the organic thin layer may be changed with the application of a voltage. In general, the memory device may be referred to as an organic or polymer memory, which enables the formation of a flexible memory device on a flexible substrate at low cost.
However, a change in the resistance of an organic thin layer may not be fully comprehended. Also, current research results propose a different opinion that the change in the resistance of the organic thin layer results from the storage of charges mentioned above as the second technique rather than the characteristics of an organic material. Thus, a doubt is being thrown on the feasibility of an organic or polymer memory device. Furthermore, according to conventional research results, although continuous attempts are being made at embodying a memory operation using various materials, it has been reported that the memory operation using the various materials are seriously problematic in terms of operation reproducibility, reliability, and environmental tolerance. Therefore, much more research on embodying flexible memory devices using the above-described method is required from now on.
Fourth, a ferroelectric thin layer may be used as a gate insulating layer of a TFT. Thus, a threshold voltage of the TFT may be changed according to a voltage application direction using the remnant polarization of the ferroelectric thin layer to embody a memory operation. Alternatively, a ferroelectric thin layer may be inserted between upper and lower conductive electrode layers to constitute a ferroelectric capacitor. Thus, a memory operation may be embodied using a difference in current of reversal of polarization caused by a variation in the remnant polarization of the ferroelectric thin layer with the voltage application direction.
The above-described technique may be typically referred to as a ferroelectric memory developed as a part of advanced nonvolatile memory technology in conventional silicon electronics. The ferroelectric memory device may be manufactured using a very simple process by adopting a transparent or flexible TFT as it is and replacing a process of forming a gate insulating layer by a process of forming a ferroelectric thin layer or forming a ferroelectric thin layer between conductive electrode layers. Furthermore, a memory device may be easily designed on relatively physically predictable operating principles based on the remnant polarization of a ferroelectric material.
However, an oxide-based ferroelectric thin layer should undergo a crystallization process so that a memory device can operate using predetermined ferroelectric characteristics. Since the oxide-based ferroelectric thin layer may be crystallized at a temperature of about 500° C. or higher, the oxide-based ferroelectric layer may be incompatible in terms of processes with transparent and flexible TFTs manufactured at temperatures of about 300° C. or lower.
Moreover, although an organic ferroelectric thin layer may be employed, the organic ferroelectric thin layer may generally cause a large leakage current to preclude formation of a thin layer. In addition, the organic ferroelectric thin layer may have little tolerance to chemicals or device fabrication processes and cannot be easily put to practical use.